”Believe in yourself as much as we believe in you,” the state secretary urged.

Kerry was in the EU capital for an international conference on Afghanistan.

The state department is trying to act as a cheerleader for the EU to limit the fallout from the UK’s decision to leave and to broker an amicable divorce between its allies.

”Our goal, as Brexit talks are launched next spring, is to prove pundits wrong and put to shame those that declared the EU dead,” Kerry said.

”We need the strongest possible EU, the strongest possible UK, and a highly integrated, collaborative relationship between them,” he added.

Kerry said the EU and the US were at risk from populists who were using xenophobia and nationalism, ideas which he said had “a bad history, a 20th century history”, to build “unrealistic political platforms”.

Tax justice

He said people’s legitimate concerns about globalisation, financial inequality, and terrorism, must be addressed however.

”We have to fight these [populist] ideas with better ones that provide jobs and implement necessary reforms,” the US secretary said.

He said the draft EU-US free trade pact could help in that regard.

”The Transatlantic Trade and Investment Partnership - TTIP - is not just about giving an additional boost to commerce,” he said. ”Our teams are sending a signal to the rest of the world on the need for high labour and environmental standards.”

The trade deal, already three years in the making, is under fire from left-wing politicians and activists.

They say it will erode welfare standards, but Kerry said trade ”was not the culprit" - unfair tax systems were.

”We need to share the benefits of trade by domestic and political choices in each of our countries, not by ending trade but through tax policies, social policies, healthcare and education policies”, he said.

He said corruption also fed anti-EU and anti-US feeling.

”Few things can be more discouraging to a young person than the conviction that the top steps of the economic ladders are reserved for the unscrupulous,” he said.

His remarks came despite the fact the US is trying to torpedo EU efforts to make US tech giant Apple pay tax in Europe.

Syria talks

He struck a darker note on the Syria war.

Washington on Monday suspended talks with Moscow over a ceasefire deal, saying Russia had flouted commitments by continuing to fighting alongside the Syrian regime, which the US accused of using chlorine gas.

"We are not giving up on the Syrian people and we are not abandoning the pursuit of peace," Kerry said.

He said Russia would have to behave differently if it was serious about a peace deal.

"Russia knows exactly what it needs to do in order to get that cessation implemented and in a fair and reasonable way," he said.